In some cases I've seen cron die when confronted with a "long" command line, so the first thing that I would suggest is to put your dd command into a basic script like:
dd if=/dev/sdb of=/dev/hda conv=notrunc,noerror
Then you can add it to root's crontab with the crontab -e command. The new line should look something like this:
* * * * * /path/to/ddscript
Each of the * characters is a place holder for a time, minute of the hour, hour of the day, day of the month, month, day of the week. The wikipedia entry for crontab has a good description of how to set the execution time.